Method and system for a high capacity cable network

ABSTRACT

A cable modem termination system (CMTS) may communicate with a plurality of cable modems using a plurality of orthogonal frequency division multiplexed (OFDM) subcarriers. The CMTS may determine a performance metric of each of the cable modems. For each of the OFDM subcarriers and each of the cable modems, the CMTS may select physical layer parameters to be used for communication with that cable modem on that OFDM subcarrier based on a performance metric of that cable modem. The parameters may be selected for each individual modem and/or each individual subcarrier, or may be selected for groups of modems and/or groups of subcarriers. The parameters may include, for example, one or more of: transmit power, receive sensitivity, timeslot duration, modulation type, modulation order, forward error correction (FEC) type, and FEC code rate.

PRIORITY CLAIM

This patent application is a continuation of U.S. patent applicationSer. No. 13/948,401, filed Jul. 23, 2013 (now U.S. Pat. No. 9,178,765)and makes reference to, claims priority to and claims benefit from U.S.Provisional Patent Application Ser. No. 61/674,733 titled “Method andSystem for a High Capacity Television Network” and filed on Jul. 23,2012, now expired. Each of the above mentioned documents is herebyincorporated herein by reference.

INCORPORATION BY REFERENCE

This application also makes reference to:

-   U.S. patent application Ser. No. 13/553,328 titled “Method and    System for Client-Side Message Handling in a Low-Power Wide Area    Network,” and filed on Jul. 19, 2012;-   U.S. patent application Ser. No. 13/485,034 titled “Method and    System for Server-Side Message Handling in a Low-Power Wide Area    Network,” and filed on May 31, 2012;-   U.S. Pat. No. 8,711,750 titled “Method and System for a Low-Power    Client in a Wide Area Network,” and issued on Apr. 29, 2014;-   U.S. Pat. No. 8,687,535 titled “Method and System for Server-Side    Handling of a Low-Power Client in a Wide Area Network,” and issued    on Apr. 1, 2014;-   U.S. Pat. No. 9,043,855 titled “Method and System for Noise    Suppression in a Cable Network,” and issued on May 26, 2015; and-   U.S. patent application Ser. No. 13/948,444 titled “Method and    System for Service Group Management in a Cable Network,” and filed    on Jul. 23, 2013.

The entirety of each of the above-mentioned applications is herebyincorporated herein by reference.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to cable/DOCSIS networks.More specifically, certain embodiments of the invention relate to amethod and system for a high-capacity cable/DOCSIS network.

BACKGROUND OF THE INVENTION

Conventional cable/DOCSIS networks can be inefficient and haveinsufficient capacity. Further limitations and disadvantages ofconventional and traditional approaches will become apparent to one ofskill in the art, through comparison of such systems with some aspectsof the present invention as set forth in the remainder of the presentapplication with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

System and methods are provided for a high-capacity cable/DOCSISnetwork, substantially as shown in and/or described in connection withat least one of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram of an example cable/DOCSIS network.

FIG. 2A depicts an example method of determining locations of CMs withinthe HFC network.

FIGS. 2B-2D depict SNR versus frequency plots for an examplecable/DOCSIS network.

FIG. 2E depicts per-channel/subcarrier selection of communicationparameters based on measured performance metrics of a plurality of cablemodems.

FIG. 3 illustrates example transmissions in an OFDM cable/DOCSIS networkin which different values of one or more physical layer communicationparameters can be used for different transmissions.

FIG. 4 illustrates an example implementation in which error-correctingbits are sent on higher-SNR subcarriers to compensate for receptionerrors on lower-SNR subcarriers.

DETAILED DESCRIPTION OF THE INVENTION

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. As another example,“x, y, and/or z” means any element of the seven-element set {(x), (y),(z), (x, y), (x, z), (y, z), (x, y, z)}. As utilized herein, the term“exemplary” means serving as a non-limiting example, instance, orillustration. As utilized herein, the terms “e.g.,” and “for example”set off lists of one or more non-limiting examples, instances, orillustrations. As utilized herein, circuitry is “operable” to perform afunction whenever the circuitry comprises the necessary hardware andcode (if any is necessary) to perform the function, regardless ofwhether performance of the function is disabled, or not enabled, by someuser-configurable setting.

FIG. 1 is a diagram of an example cable/DOCSIS network. The examplenetwork comprises a cable modem termination system (CMTS) 102, a fibernode 104, amplifiers 106 ₁-106 ₃, a directional coupler 108, splitters110 ₁-110 ₃, and cable modems (CMs) 112 ₁-112 ₅.

The CMTS 102 may comprise circuitry operable to manage connections tothe CMs 112 ₁-112 ₅. This may include, for example: participating inranging operations to determine physical layer parameters used forcommunications between the CMTS 102 and CMs 112 ₁-112 ₅; forwarding ofdynamic host configuration protocol (DHCP) messages between a DHCPserver and the CMs 112 ₁-112 ₅; forwarding of time of day messagesbetween a time of day server and the CMs 112 ₁-112 ₅; directing trafficbetween the CMs 112 ₁-112 ₅ other network devices (e.g., Ethernetinterfaces of the CMTS 102 may face the Internet, Optical RF interfacesof the CMTS 102 may face the CMs, and the CMTS may direct trafficbetween and among the Ethernet and Optical RF interfaces); and managingregistration of the CMs 112 ₁-112 ₅ to grant the cable modems network(e.g., Internet) access. The registration process for a CM 112 _(X) (Xbetween 1 and 5 for the example network of FIG. 1) may comprise the CM112 _(X) sending a registration request along with its configurationsettings, and the CMTS 102 accepting or rejecting the cable modem basedon the configuration settings. The registration process may additionallycomprise an exchange of security keys, certificates, or otherauthentication information.

The fiber node 104 may comprise circuitry operable to convert betweenoptical signals conveyed via the fiber optic cable 103 and electricalsignals conveyed via coaxial cable 105.

Each of the amplifiers 106 ₁-106 ₃ may comprise a bidirectionalamplifier which may amplify downstream signals and upstream signals,where downstream signals are input via upstream interface 107 a andoutput via downstream interface 107 b, and upstream signals are inputvia downstream interface 107 b and output via upstream interface 107 a.The amplifier 106 ₁, which amplifies signals along the main coaxial“trunk” may be referred to as a “trunk amplifier.” The amplifiers 106 ₂and 106 ₃ which amplify signals along “branches” split off from thetrunk may be referred to as “branch” or “distribution” amplifiers.

The directional coupler 108 may comprise circuitry operable to directdownstream traffic incident on interface 109 a onto interfaces 109 b and109 c, and to direct upstream traffic incident on interfaces 109 b and109 c onto interface 109 a. The directional coupler 108 may be a passivedevice.

Each of the splitters 110 ₁-110 ₃ may comprise circuitry operable tooutput signals incident on each of its interfaces onto each of its otherinterfaces. Each of the splitters 110 ₁-110 ₃ may be a passive device.

Each of the cable modems (CMs) 112 ₁-112 ₅ may comprise circuitryoperable to communicate with, and be managed by, the CMTS 1102 inaccordance with one or more standards (e.g., DOCSIS). Each of the CMs112 ₁-112 ₅ may reside at the premises of a cable/DOCSIS subscriber.

The components (including, fiber optic cables, coaxial cables,amplifiers, directional couplers, splitters, and/or other devicesbetween the CMTS 102 and the CMs 112 ₁-112 ₅ may be referred to as ahybrid fiber coaxial (HFC) network. Any of the amplifiers, directionalcoupler, and splitters may be referred to generically as a couplingdevice.

FIG. 2A depicts an example method of determining locations of CMs withinthe HFC network. Determining the locations may include determining oneor more measured performance metrics for any particular CM 112X or groupof CMs. A measured performance metric may be, for example, anSNR-related metric such as noise levels, strength of received desiredsignals, SNR at a particular frequency, SNR over a range of frequencies(an SNR profile), bit error rate, symbol error rate, and/or the like.Although various implementations using SNR-profile as the pertinentperformance metric are described herein, other implementations may use adifferent metric. As shown in FIG. 2A, to determine one or more measuredperformance metric(s) for any particular CM 112 _(X) or group of CMs,the CMTS 102 may transmit, at time 1, a message 202 that is destined(unicast, multicast, or broadcast) for the CM(s) and that functions as aprobe to enable determination of the metric(s) for the CMs. The message202 may be sent on multiple channels spanning multiple frequencies.Similarly, where OFDM is used for communications between the CMTS 102and the CM(s), the message 202 may be transmitted on each subcarrier, ormay be sent on a subset of subcarriers and then interpolation may beused for determining the metric(s) of subcarriers on which the message202 was not sent.

The message 202 may be transmitted with such encoding, modulation, andtransmit power such that even a CM 112 _(X) with a worst-caseperformance metric(s) can receive the message and accurately measure themetric(s). In this regard, FIG. 2B shows a SNR versus frequency graphfor an example HFC network that uses eight channels/subcarriers. Theline 222 in FIG. 2B represents a composite worst-case SNR profile forone or more CM(s) in the HFC network to which the message 202 isdestined. For example, line 222 may be a SNR profile for a single CM 112_(X) to which the message 202 is to be unicast. As another example, theline 222 may be a composite worst-case SNR profile for a plurality ofCMs 112 of a particular service group to which the message 202 is to bemulticast. As another example, the line 222 may be a compositeworst-case SNR profile for all CMs of an HFC network handled by the CMTS102 to which the message 202 is to be broadcast. The message 202 may betransmitted such that the minimum SNR needed to receive and accuratelymeasure the SNR profile is below the line 222 (e.g., SNR needed forreceiving the message 202 may be the line 224).

Upon receipt of the message 202, a CM 112 _(X) may measure, over thechannels/subbands on which the message was sent, one or more metrics(e.g., SNR versus frequency profile) for the transmission 202. The CM112 _(X) may then report the metric(s) back to the CMTS 102 via amessage 204. In an example implementation, the message 202 may containinformation about when and/or how the CM(s) are supposed to report theirmetric(s) (e.g., SNR profiles) back to the CMTS 102. In this regard, themessage 202 may contain information that is the same as and/or oranalogous to what may be found in a MAP, UCD, and/or other MACmanagement message defined in a DOCSIS standard. Accordingly, themessage 202 may have specified a format of the message 204 and that themessage 204 is to be transmitted at time T+Δ.

Once the metric(s) of one or more CM(s) are known to the CMTS 102,physical layer communication parameters to be used for communicationsbetween the CMTS 102 and the CM(s) may be determined based on themetric(s). Physical layer parameters may be configured/coordinated usingupstream and/or downstream MAP messages, upstream channel descriptors(UCDs), other MAC management messages defined in DOCSIS protocols,and/or purpose-specific messages tailored to configuring the parametersbased on one or more measured performance metric(s) as described in thisdisclosure. Physical layer communication parameters may be determinedper CM based on each CM's respective metric(s) (e.g., each CM's SNRprofile), per-service group based on a composite metric(s) of the CM(s)assigned to that service group (e.g., composite SNR profile for theCM(s) of that service group), per physical region of the HFC networkbased on a composite metric of the CMs located in that physical region(e.g., composite SNR profile for the CM(s) in that physical region),and/or the like. Furthermore, once the metric(s) of a CM 112 _(X) isdetermined, the CMTS 102 may assign that CM 112 _(X) to one or moreservice groups based on its metric(s). Example physical layer parametersinclude: encoding parameters, modulation parameters, transmit power,receive sensitivity, timeslot duration, channel(s) or subcarrier(s) onwhich to listen, channel(s) or subcarrier(s) on which to transmit,and/or the like. Example encoding parameters include: type of forwarderror correction (FEC) to be used (e.g., Reed-Solomon, LDPC, etc.), FECblock size, FEC code rate, etc. Example modulation parameters include:type of modulation (e.g., frequency shift keying (FSK), phase shiftkeying (PSK), quadrature amplitude modulation (QAM), etc.), modulationdepth, modulation order, etc.

In an example implementation, the transmission of messages 202, thecalculation of the metrics, such as an SNR profile, by the CM(s), thetransmission 204, and subsequent configuration of physical layerparameters based on the metric(s) may take place in parallel with otheroperations performed during the registration/ranging process.

Referring now to FIG. 2C, there is again shown the line 222 whichrepresents an applicable SNR profile (e.g., an individual SNR profile ifconfiguring physical layer parameters per CM, a composite SNR profilefor a service group if configuring physical layer parameters per servicegroup, or a composite SNR profile for a particular physical region ifconfiguring physical layer parameters based on physical location withinthe HFC network). Also shown is a line 226 corresponding to SNRutilization for communications with the CM(s) associated with theprofile 222. Assuming the distance 228 is the minimum desired headroom(e.g., to allow for noise, etc.), then the physical layer communicationparameters resulting in line 226 are nearly optimal in the sense thatthere is minimal headroom on each of channels/subbands 1, 3, 4, 6, 7, 8,and only slightly more than minimal headroom on channels/subbands 2 and5.

FIG. 2D illustrates example SNR profiles for the network of FIG. 1. Thelight solid line 236 represents the SNR profile of CM 112 ₁, the dashedline 222 represents the SNR profiles of CMs 112 ₂ and 112 ₃ (which areassumed to be the same for simplicity of illustration), the heavy solidline represents the reported SNR profiles of CMs 112 ₄ and 112 ₅ (whichare assumed to be the same for simplicity of illustration). The SNRprofiles of 112 ₄ and 112 ₅ may be lower than the others because, forexample, higher device and/or cable losses (e.g., as a result ofpoor-performing coupling devices and/or longer cables). In an exampleimplementation, given the profiles 232, 234, and 236, the CMTS 102 mayreserve lower-frequency channels/subcarriers for communications with CMs112 ₄ and 112 ₅ and may reserve higher-frequency channels/subcarriersfor communications with from CMs 112 ₁, 112 ₂, and 112 ₃. In thisregard, in this example implementation, SNR falls off as the distance tothe CMTS 102 increases and falls off faster at higher frequencies thanat lower frequencies, resulting in Δ2>Δ4 and Δ1>Δ3. Thus, whereas theSNR of CMs 112 ₄ and 112 ₅ in channel/subcarrier 1 is only Δ4 less thanthe SNR of CM 112 ₁ in channel/subcarrier 1, the SNR of CMs 112 ₄ and112 ₅ in channel/subcarrier 8 is Δ2 less than the SNR of CM 112 ₁ inchannel/subcarrier 8. Thus, by using channel/subcarrier 8 for CM 112 ₁and channel/subc1 for CMs 112 ₄ and 112 ₅, there is a net SNR increaseof Δ2−Δ4 (i.e., communications to CM 112 ₁ lose Δ4 in SNR butcommunications to CMs 112 ₄ and 112 ₅ gain Δ2 in SNR).

FIG. 2E illustrates per-channel/subcarrier selection of communicationparameters based on measured SNR (as an example of a measuredperformance metric) in an OFDM CATV network. Each of the graph 242 _(M)(M between 1 and 8) is a histogram showing, for a hypothetical HFCnetwork comprising 20 cable modems and using 8 channels/subcarriers, thenumber of CMs modems that reported each SNR value on subcarrier M. Forsimplicity of illustration, the SNR levels in FIG. 2E are normalized andrange from 1 to 16 (the number 16 was chosen arbitrarily and isnon-limiting). In the example implementation, the average SNR isrelatively-high for low-frequency channels/subcarriers (thelowest-frequency being channel/subcarrier 1) and decreases as thefrequency increases (the highest-frequency being channel/subcarrier 8).In other example implementations, noise, imperfections, and/or othercharacteristics of the CATV network may result in SNR measurements whichdo not necessarily decrease monotonically across channels/subcarriers.

The CMTS 102 may utilize the data corresponding to graphs 242 ₁-242 ₈shown when determining physical layer communication parameters to beused for each of the channels/subcarriers. In an example implementation,the physical layer parameters for a channel/subcarrier may be selectedsuch that all CMs in the service group can successfully receivetransmissions to the service group. Lower SNR may require, for example,lower modulation order, lower FEC code rate, and/or higher transmitpower. In such an implementation, the graphs in FIG. 2E may result inthe following selection of physical layer parameters: for subcarrier 1,parameters that enable reception with a tolerable amount of errors at anSNR of 7; for subcarrier 2, parameters that enable reception with atolerable amount of errors at an SNR of 6; for subcarrier 3, parametersthat enable reception with a tolerable amount of errors at an SNR of 4;for subcarrier 4, parameters that enable reception with a tolerableamount of errors at an SNR of 6; for subcarrier 5, parameters thatenable reception with a tolerable amount of errors at an SNR of 3; forsubcarrier 6, parameters that enable reception with a tolerable amountof errors at an SNR of 2; for subcarrier 7, parameters that enablereception with a tolerable amount of errors at an SNR of 3; forsubcarrier 8, parameters that enable reception with a tolerable amountof errors at an SNR of 1.

In another example implementation, for each channel/subcarrier, theparameters may be selected such that reception on the subcarrierrequires a value of a metric (e.g., SNR value) that a predeterminednumber or percentage of the CMs reported they are unable to achieve. Inthis manner, other CMs may receive higher throughput at the expense ofsome CMs having a higher number of receive errors. The errors for theCMs that have insufficient metric value for the selected parameters maybe compensated through unicast and/or multicast transmission ofreplacement data and/or additional error correction bits as discussedbelow. For example, assuming that it is acceptable for five of thesixteen CMs in FIG. 2E to fall below the threshold SNR valuecorresponding to the selected physical layer parameters, the graphs inFIG. 2E may result in the following selection of physical layerparameters: for subcarrier 1, parameters that enable reception with atolerable amount of errors at an SNR of 12; for subcarrier 2, parametersthat enable reception with a tolerable amount of errors at an SNR of 11;for subcarrier 3, parameters that enable reception with a tolerableamount of errors at an SNR of 10; for subcarrier 4, parameters thatenable reception with a tolerable amount of errors at an SNR of 9; forsubcarrier 5, parameters that enable reception with a tolerable amountof errors at an SNR of 8; for subcarrier 6, parameters that enablereception with a tolerable amount of errors at an SNR of 7; forsubcarrier 7, parameters that enable reception with a tolerable amountof errors at an SNR of 6; for subcarrier 8, parameters that enablereception with a tolerable amount of errors at an SNR of 5.

In an example implementation, hierarchical modulation may be used withcoarse bits/most significant bits (MSBs) used for sensitive and/orcritical information (e.g., control messages such as upstream and/ordownstream MAPS, sync messages, packet headers, I frames of an MPEGstream, and/or the like) and finer bits/least significant bits (LSBs)used for less sensitive information (e.g., B frames of an MPEG stream).In an example implementation, hierarchical modulation may be used withcoarse bits or MSBs utilized for broadcast messages and finer bits orLSBs used for unicast and/or multicast messages to those CMs that havesufficient metric value(s) (e.g., SNR value) to detect the finer bits.In an example implementation, hierarchical modulation may be used whereMSBs and LSBs are transmitted on subcarriers that have higher SNR andonly MSBs may be transmitted on subcarriers which have lower SNR.Subcarriers on which hierarchical modulation is used may be thosesubcarriers for which the disparity between best-case SNR (e.g., SNRreported by a CM that is closest to the CMTS) and worst-cast SNR (e.g.,SNR reported by CM that is furthest from the CMTS, that is on aparticularly noisy branch, that is behind a defective cable or couplingelement, etc.) is above a threshold.

FIG. 3 illustrates example transmissions in an OFDM cable/DOCSIS networkin which different values of one or more physical layer communicationparameters can be used for different transmissions. The transmissionsshown may be coordinated using, for example, upstream and/or downstreamDOCSIS MAP messages. The duration of the timeslot and/or which timeslota transmission is scheduled for may be controlled based on latencyrequirements of the transmission and/or other transmissions in thenetwork.

The transmission 301, which may be to and/or from one or more CM(s) of afirst service group, uses all subcarriers, a timeslot of duration T1,and different physical layer parameters (“parms M” where M correspondsto the subcarrier index) for each of the eight subcarriers. Thetransmission 302, which may be to and/or from one or more CM(s) of asecond service group, uses only the three highest-frequency subcarriers,a timeslot of duration T1+T2, and uses the same physical layerparameters (“parms 8 ”) on each of the three subcarriers. Thetransmission 303 uses the lower five subcarriers, a timeslot of durationT1, and a different set of physical layer parameters (“parms M” where Mcorresponds to the subcarrier index) on each of the five subcarriers.The transmission 304 uses the lower five subcarriers, a timeslot ofduration T2, and a two sets of physical layer parameters (“parms 4 ” and“parms 2 ”). The transmission 305 uses only the lowest frequencysubcarrier, a timeslot of duration 2T1+2T2, and a single set of physicallayer parameters. During time interval over which transmission 305 takesplace, circuitry of the CMTS 102 and/or the CMs which may be requiredfor receiving on the seven higher-frequency subcarriers may be powereddown to conserve energy. The transmission 406 uses the all of thesubcarriers, a timeslot of duration T2+T1, and the same physical layerparameters (“parms 5 ”) on each of the subcarriers.

In an example implementation, the physical layer parameters may beconfigured to enable sleep duty cycling of CMs and/or other componentsof the network. For example, the physical layer parameters may beconfigured to maximize throughput while minimizing transmission timesuch that more time can be spent in a sleep/low-power mode.

FIG. 4 illustrates an example implementation in which error-correctingbits are sent on higher-SNR subcarriers to compensate for receptionerrors on lower-SNR subcarriers. In FIG. 4, the line 406 corresponds tothe SNR profile of a cable modem 112 _(X), and line 408 corresponds tothe SNR required on each subcarrier in order to successfully receivebroadcast transmissions (i.e., receive with a tolerable number oferrors). For simplicity of illustration, the required SNR is flat acrossthe subcarriers. In the implementation depicted, CM 112 _(X) has excessSNR in channels/subcarriers 1-4 (the excess capacity corresponding toarea 404), but has inadequate SNR in channels/subcarrier 5-8 (thecapacity deficit corresponding to area 410). Consequently, data receivedon channel/subcarriers 5-8 may have unacceptably high error rates. Tocompensate for the deficiency on channels/subcarriers 5-8, replacementdata and/or additional parity bits for the packets transmitted onsubcarriers 5-8 may be transmitted on subcarriers 1-4 (e.g., in the formof a unicast transmission, or multicast transmission where other CMsalso have a deficit on the same subcarriers). CM 112 _(X) may receive abroadcast packet, process the broadcast packet to recover a portion of amessage, receive a unicast or multicast packet containing an additionaland/or replacement portion(s) of the message, process the unicast and/ormulticast packet to recover the additional and/or replacementportion(s), and combine the additional and/or replacement portions withthe portion recovered from the broadcast packet to reconstruct themessage with an acceptable number of errors.

In an example implementation, a cable modem termination system (CMTS)(e.g., 102) may communicate with a plurality of cable modems (e.g., 112₁-112 ₅) using a plurality of orthogonal frequency division multiplexed(OFDM) subcarriers (e.g., subcarriers 1-8 of FIG. 4). The CMTS maydetermine a performance metric of each of the cable modems. For each oneof the plurality of OFDM subcarriers and each one of the cable modems,the CMTS may select physical layer parameters to be used forcommunication with the one of the cable modems on the one of the OFDMsubcarriers based on a performance metric of the one of the cablemodems. Parameters may be selected for each individual cable modem(e.g., a separate selection for each of CMs 112 ₁-112 ₅) or for groupsof cable modems (e.g., a first selection for CMs 112 ₁-112 ₃ and asecond selection for CMs 112 ₄ and 112 ₅). The performance metric may bea signal-to-noise ratio (SNR) profile across the OFDM subcarriers. Theplurality of cable modems may belong to a single service group, and thephysical layer parameters may be selected such that a threshold SNRrequired for receiving a packet broadcast to the service group is higherthan a subset of the SNR profiles corresponding to a subset of the cablemodems. The contents of the broadcast packet may be fully or partiallyretransmitted to the subset of modems in a unicast or multicast packet.

Continuing with this example implementation, the selecting the physicallayer parameters may include selecting whether to use hierarchicalmodulation for communication with the one of the cable modems. The CMTSmay select use of hierarchical modulation for a particular one of theOFDM subcarriers when a difference between a performance metric measuredfor a first one of the cable modems on the particular one of the OFDMsubcarriers and a the performance metric measured for a second one ofthe cable modems on the particular one of the OFDM subcarriers isgreater than a predetermined amount. The CMTS may communicate with thecable modems using hierarchical modulation, wherein both of moresignificant bits and less significant bits are transmitted on a firstsubset of the OFDM subcarriers for which each of the modems has arelatively-high value of a performance metric, and only more significantbits are transmitted on a second subset of the OFDM subcarriers forwhich one or more of the modems has a relatively-low of a performancemetric.

Continuing with this example implementation, the CMTS may select thephysical layer parameters such that there is excess capacity on a firstsubset of the OFDM subcarriers and a capacity deficit on a second set ofthe OFDM subcarriers. The CMTS may transmit, on the first subset of theOFDM subcarriers, parity bits for packets sent on the second subset ofthe OFDM subcarriers. The CMTS may transmit, on the first subset of theOFDM subcarriers, replacement bits for packets sent on the second subsetof the OFDM subcarriers. The physical layer parameters may include oneor more of: transmit power, receive sensitivity, timeslot duration,modulation type, modulation order, forward error correction (FEC) type,and FEC code rate.

Other embodiments of the invention may provide a non-transitory computerreadable medium and/or storage medium, and/or a non-transitory machinereadable medium and/or storage medium, having stored thereon, a machinecode and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for a highcapacity CATV network.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputing system, or in a distributed fashion where different elementsare spread across several interconnected computing systems. Any kind ofcomputing system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computing system with a program orother code that, when being loaded and executed, controls the computingsystem such that it carries out the methods described herein. Anothertypical implementation may comprise an application specific integratedcircuit or chip.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method comprising: in a cable modem terminationsystem (CMTS) which communicates with a plurality of cable modems usinga plurality of orthogonal frequency division multiplexed (OFDM)subcarriers, executing instructions for: selecting physical layerparameters to be used for communication such that there is excesssignal-to-noise ratio (SNR), relative to a threshold SNR required forsaid communication, on a first subset of said OFDM subcarriers, and aSNR deficit, relative to said threshold SNR, on a second set of saidOFDM subcarriers, wherein said selecting said physical layer parametersincludes selecting whether to use hierarchical modulation for saidcommunication with said one of said cable modems; and transmitting, onsaid first subset of said OFDM subcarriers, error correction data forpackets sent on said second subset of said OFDM subcarriers.
 2. Themethod of claim 1, comprising transmitting, on said first subset of saidOFDM subcarriers, parity bits for packets sent on said second subset ofsaid OFDM subcarriers.
 3. The method of claim 1, comprisingtransmitting, on said first subset of said OFDM subcarriers, replacementbits for packets sent on said second subset of said OFDM subcarriers. 4.The method of claim 1, wherein said physical layer parameters includeone or more of: transmit power, receive sensitivity, timeslot duration,modulation type, modulation order, forward error correction (FEC) type,and FEC code rate.
 5. A system comprising: a cable modem terminationsystem (CMTS); and a memory storing instructions which when executedcause the CMTS to: communicate with a plurality of cable modems using aplurality of orthogonal frequency division multiplexed (OFDM)subcarriers; determine a performance metric of each of said cablemodems; for each one of said plurality of OFDM subcarriers and each oneof said cable modems, select physical layer parameters to be used forcommunication with said one of said cable modems on said one of saidOFDM subcarriers based on a performance metric of said one of said cablemodems, wherein: said performance metric is a signal-to-noise ratio(SNR) profile across said OFDM subcarriers; said plurality of cablemodems belong to a single service group; and said physical layerparameters are selected such that a threshold SNR required for receivinga packet broadcast to said service group is higher than a subset of saidSNR profiles corresponding to a subset of said cable modems wherein theselected physical layer parameters includes selection of whether to usehierarchical modulation for said communication with said one of saidcable modems; and transmit, on said first subset of said OFDMsubcarriers, error correction data for packets sent on said secondsubset of said OFDM subcarriers.
 6. A system comprising: a cable modemtermination system (CMTS); and a memory storing instructions which whenexecuted cause the CMTS to: communicate with a plurality of cable modemsusing a plurality of orthogonal frequency division multiplexed (OFDM)subcarriers; and select physical layer parameters to use forcommunication such that there is excess signal-to-noise ratio (SNR),relative to a threshold SNR required for said communication, on a firstsubset of said OFDM subcarriers and a SNR deficit, relative to saidthreshold SNR, on a second set of said OFDM subcarriers wherein saidselect physical layer parameters includes selection of whether to usehierarchical modulation for said communication with said one of saidcable modems; and transmit, on said first subset of said OFDMsubcarriers, error correction data for packets sent on said secondsubset of said OFDM subcarriers.
 7. The system of claim 6, wherein saidCMTS is operable to transmit, on said first subset of said OFDMsubcarriers, parity bits for packets sent on said second subset of saidOFDM subcarriers.
 8. The system of claim 6, wherein said CMTS isoperable to transmit, on said first subset of said OFDM subcarriers,replacement bits for packets sent on said second subset of said OFDMsubcarriers.
 9. The system of claim 6, wherein said physical layerparameters include one or more of: transmit power, receive sensitivity,timeslot duration, modulation type, modulation order, forward errorcorrection (FEC) type, and FEC code rate.
 10. The system of claim 5,wherein said physical layer parameters include one or more of: transmitpower, receive sensitivity, timeslot duration, modulation type,modulation order, forward error correction (FEC) type, and FEC coderate.